Ancient fossils have evolution's first shells

A series of spectacularly preserved, 750-million-year-old fossils represent
the microscopic origins of biomineralisation, or the ability to
convert minerals into hard, physical structures. This process is
what makes bones, shells, teeth and hair possible, literally
shaping the animal kingdom and
even Earth itself.

The fossils were pried from ancient rock formations in Canada's
Yukon by earth scientists Francis Macdonald and Phoebe Cohen of
Harvard University. In a June Geology paper,
they describe their findings as providing "a unique window into the
diversity of early eukaryotes."

That window opens into an evolutionary
period less celebrated than the kaleidoscopic radiations of
the Cambrian, but in its own way no less impressive. The simple
single-celled organisms that dominated life's first few billion
years were rapidly becoming more complex, building a store of
innovations that sustained some through the so-called Snowball
Earth period, when Earth's climate turned so cold that the equator
resembled Antarctica.

One such innovation was biomineralisation, though evidence for
its occurrence at this time was inconclusive. Using molecular
clocks and genetic
trees to reverse-engineer evolutionary histories, previous
research placed the beginning of biomineralisation at
about 750 million years ago. Around that time, the fossil record
gets suggestive, turning up vase-shaped amoebas with
something like scales in their cell walls, algae with cell
walls possibly made from calcium carbonate and
sponge-like creatures with seemingly mineralised bodies. But
in each of these examples, caveats abound. What appears to be
biomineralisation might be a fossil illusion produced as soft
tissue turned to stone.

In the new study, Cohen and Macdonald examined hundreds of
fossils under microscopes. They found three common species of
algae, dubbed Archeoxybaphon,
Bicorniculum and Characodictyon,
bearing mineral traces suggestive of biological origins. Crucially,
the shapes of these organisms didn't vary between specimens. The
fossils of soft-bodied creatures, by contrast, tend to be distorted
by compaction.

Once identified, these standard-bearers for biomineralisation
raise a basic question: Why bones at all? After all, life did
perfectly well for three billion years without them.

In a commentary accompanying the finding, paleontologist
Susannah Porter of the University of California, Santa Barbara
hazards an explanation: Bones evolved as a defense against
predators. That's the best guess for why, 200 million years later,
skeletons evolved independently in at least two dozen separate
animal clades. The same basic dynamics should apply to
single-celled organisms, too.

Indeed, there's evidence for fierce predation in the
single-celled world, with fossils of 1.2 billion-year-old
protists containing photosynthetic structures almost certainly
acquired by gobbling algae. In this light, biomineralisation would
seem to be a defense mechanism, a way of sticking in a predator's
craw or deflecting a stinger.

Of course, predators eventually developed their own
biomineralisation strategies, as did other algae. Eventually it
became ubiquitous in the marine world, to the point where what we
now call limestone is simply a composite of microscopic fossil
seashells. It's also the primary ingredient in concrete. Their
shells have become our own.